Polyesters represented by polyethylene terephthalate (PET), polybutylene terephthalate (PBT), polyethylene naphthalate (PEN) etc. are excellent in mechanical and chemical characteristics, and are used in various fields for example in fibers for clothing and industrial materials, films for packaging or for magnetic tapes, sheets, bottles such as hollow molded articles, casings for electrical or electronic parts, and other molded articles of engineering plastics, depending on the characteristics of each polyester.
As typical polyester, polyester comprising an aromatic dicarboxylic acid and an alkylene glycol as major constituent components, for example polyethylene terephthalate (PET), is industrially produced by esterification or transesterification of terephthalic acid or dimethyl terephthalate and ethylene glycol to produce bis(2-hydroxyethyl)terephthalate which is then subjected to polycondensation at high temperatures in vacuo in the presence of a catalyst.
As a conventional polyester polymerization catalyst used in polycondensation of polyester, antimony trioxide has been used widely. Antimony trioxide is an inexpensive and highly active catalyst, but when antimony trioxide is used as a major component, that is, when it is used in such an amount as to exhibit a practical rate of polymerization, an antimony metal is precipitated at the time of polycondensation to cause problems such as gray discoloration or formation of insoluble particles in polyester. For this reason, polyester absolutely free of antimony or excluding antimony as a major catalytic component is desired.
The above-described insoluble particles in polyester cause the following problems:    (1) In polyester for film, the antimony metal precipitated serves as insoluble particles in polyester, which causes not only contamination of an outlet during melt extrusion but also deficiency in the surface of film. Further, when the polyester with insoluble particles is used as a starting material of hollow molded articles, it is difficult to obtain hollow molded articles excellent in transparency.    (2) The insoluble particles in polyester for fibers serves as insoluble particles not only causing a reduction in the strength of fibers, but also deposits around spinnerets during spinning. In production of polyester fibers, a polyester polymerization catalyst not causing formation of insoluble particles is desired from the viewpoint of productivity.
As a method of solving the problem described above, an attempt had been made at preventing gray discoloration and formation of insoluble particles in PET while using antimony trioxide as a catalyst. In Japanese Patent No. 2666502, for example, formation of black insoluble particles in PET is prevented by using antimony trioxide, a bismuth compound and a selenium compound as a polycondensation catalyst. Further, JP-A 9-291141 describes that precipitation of an antimony metal is prevented when antimony trioxide containing sodium and iron oxides is used as a polycondensation catalyst. However, these polycondensation catalysts cannot achieve the object of reducing the content of antimony in polyester.
As a method of solving the problem of the antimony catalyst in uses requiring transparency of PET bottles etc., for example JP-A 6-279579 discloses a method of improving transparency by prescribing the proportion of antimony and phosphorus compounds used. However, it cannot be said that hollow molded articles made of polyester obtained by this method are sufficiently transparent.
Further, JP-A 10-36495 discloses a continuous process for producing polyester excellent in transparency, which comprises use of antimony trioxide, phosphoric acid and a sulfonic acid compound. However, polyester obtained by such a method has lower thermal stability, and there is the problem of a high content of acetaldehyde in the resultant hollow molded article.
Polycondensation catalysts substituted for antimony type catalysts such as antimony trioxide have also been examined, and titanium compounds such as tetraalkoxy titanate or tin compounds have previously been proposed, but there is a problem that polyester produced by using these compounds is easily thermally degraded during melt molding, and the polyester is significantly discolored.
In an attempt at solving the problem arising when such titanium compounds are used as the polycondensation catalyst, for example JP-A 55-116722 proposes a method of simultaneously using tetraalkoxy titanate in combination with a cobalt salt and a calcium salt. Further, JP-A 8-73581 proposes a method of using tetraalkoxy titanate in combination with a cobalt compound as the polycondensation catalyst and simultaneously using an optical brightener. By these techniques, PET discoloration occurring when tetraalkoxy titanate is used as the polycondensation catalyst can be reduced, but prevention of thermal decomposition of PET cannot be achieved.
In another attempt at preventing thermal degradation during melt molding of polyester polymerized in the presence of a titanium compound as the catalyst, for example JP-A 10-259296 describes a method of adding a phosphorus compound after polymerization of polyester in the presence of the titanium compound as the catalyst. However, effective mixing of the additive with the polymer after polymerization is technically difficult and leads to higher costs, so this prior art method is not practically used under the present circumstances.
A method of adding an alkali metal compound to an aluminum compound to form a polyester polymerization catalyst having a sufficient catalytic activity is also known. When such a known catalyst is used, polyester excellent in thermal stability can be obtained, but this known catalyst using an alkali metal compound in combination should be added in a larger amount in order to attain a practical catalytic activity, and as a result, insoluble particles attributable to the alkali metal compound are increased, there arises a problem that when the PET is used in fibers, the spinnability and physical properties of fibers are getting worse, and when the PET is used in films, the physical properties of the films are getting worse, and hydrolytic resistance are lowered.
As an non-antimony catalyst, having an excellent catalytic activity and giving polyester excellent in thermal stability and hydrolytic resistance, a germanium compound has been practically used, but this catalyst has a problem that it is very expensive and easily distilled away from the reaction system during polymerization, thus changing the concentration of the catalyst in the reaction system and making control of polymerization difficult, so use of the germanium component as a major catalytic component is problematic.
For preventing thermal degradation of polyester during melt molding, there is also a method of removing a catalyst from polyester. JP-A 10-251394 discloses a method of removing a catalyst from polyester wherein a polyester resin is brought into contact with an extractant as supercritical fluid in the presence of an acidic substance. However, the method of using such supercritical fluid is technically difficult and leads to higher costs for products, and is thus not preferable.
For the reasons described above, there is demand for a polymerization catalyst which comprises a metal component other than antimony and germanium as a major catalytic component, has an excellent catalytic activity and gives polyester hardly suffering thermal degradation during melt molding and excellent in thermal stability and hydrolytic resistance.
This invention provides a polyester polymerization catalyst which contains neither an antimony compound nor a germanium compound as a major catalytic component but contains aluminum as a major metal component, has an excellent catalytic activity, and without deactivating or removing the catalyst, gives polyester effectively inhibited from suffering thermal degradation during melt molding and excellent in thermal stability, thermo oxidative stability and hydrolytic resistance. Further, this invention provides polyester produced with the catalyst, which is excellent in thermal stability, thermo oxidative stability and hydrolytic resistance during melt molding of films, hollow molded articles such as bottles, and fibers and which is superior in quality level even if virgin resin is used or scraps generated during molding are reutilized, as well as a process for producing polyester by using the polyester polymerization catalyst. Another object of this invention is to provide fibers, films, sheets and hollow molded articles comprising the polyester produced with the novel polyester polymerization catalyst which contains neither a germanium compound nor an antimony compound as a major catalytic component.